Bug 1754448 - Add basic testing for snapshot re-using; r=dom-storage-reviewers,webidl,asuth,jari,ipc-reviewers,mccr8

Differential Revision: https://phabricator.services.mozilla.com/D138405

dom/localstorage/ActorsParent.cpp

@@ -1580,10 +1580,10 @@ class Datastore final
   //////////////////////////////////////////////////////////////////////////////
   // Mutation Methods
   //
-  // These are only called during Snapshot::RecvAsyncCheckpoint
+  // These are only called during Snapshot::Checkpoint
 
   /**
-   * Used by Snapshot::RecvAsyncCheckpoint to set a key/value pair as part of an
+   * Used by Snapshot::Checkpoint to set a key/value pair as part of an
    * explicit batch.
    */
   void SetItem(Database* aDatabase, const nsString& aKey,
@@ -2023,6 +2023,11 @@ class Snapshot final : public PBackgroundLSSnapshotParent {
   // Reference counted.
   ~Snapshot();
 
+  mozilla::ipc::IPCResult Checkpoint(nsTArray<LSWriteInfo>&& aWriteInfos);
+
+  mozilla::ipc::IPCResult CheckpointAndNotify(
+      nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos);
+
   void Finish();
 
   // IPDL methods are only called by IPDL.
@@ -2036,6 +2041,12 @@ class Snapshot final : public PBackgroundLSSnapshotParent {
   mozilla::ipc::IPCResult RecvAsyncCheckpointAndNotify(
       nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) override;
 
+  mozilla::ipc::IPCResult RecvSyncCheckpoint(
+      nsTArray<LSWriteInfo>&& aWriteInfos) override;
+
+  mozilla::ipc::IPCResult RecvSyncCheckpointAndNotify(
+      nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) override;
+
   mozilla::ipc::IPCResult RecvAsyncFinish() override;
 
   mozilla::ipc::IPCResult RecvSyncFinish() override;
@@ -2456,6 +2467,19 @@ class PreloadedOp : public LSSimpleRequestBase {
   void GetResponse(LSSimpleRequestResponse& aResponse) override;
 };
 
+class GetStateOp : public LSSimpleRequestBase {
+  nsCString mOrigin;
+
+ public:
+  GetStateOp(const LSSimpleRequestParams& aParams,
+             const Maybe<ContentParentId>& aContentParentId);
+
+ private:
+  nsresult Start() override;
+
+  void GetResponse(LSSimpleRequestResponse& aResponse) override;
+};
+
 /*******************************************************************************
  * Other class declarations
  ******************************************************************************/
@@ -3468,6 +3492,14 @@ PBackgroundLSSimpleRequestParent* AllocPBackgroundLSSimpleRequestParent(
       break;
     }
 
+    case LSSimpleRequestParams::TLSSimpleRequestGetStateParams: {
+      RefPtr<GetStateOp> getStateOp = new GetStateOp(aParams, contentParentId);
+
+      actor = std::move(getStateOp);
+
+      break;
+    }
+
     default:
       MOZ_CRASH("Should never get here!");
   }
@@ -5696,7 +5728,7 @@ mozilla::ipc::IPCResult Snapshot::RecvDeleteMe() {
   return IPC_OK();
 }
 
-mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpoint(
+mozilla::ipc::IPCResult Snapshot::Checkpoint(
     nsTArray<LSWriteInfo>&& aWriteInfos) {
   AssertIsOnBackgroundThread();
   // Don't assert `mUsage >= 0`, it can be negative when multiple snapshots are
@@ -5749,7 +5781,7 @@ mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpoint(
   return IPC_OK();
 }
 
-mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpointAndNotify(
+mozilla::ipc::IPCResult Snapshot::CheckpointAndNotify(
     nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) {
   AssertIsOnBackgroundThread();
   // Don't assert `mUsage >= 0`, it can be negative when multiple snapshots are
@@ -5816,6 +5848,26 @@ mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpointAndNotify(
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpoint(
+    nsTArray<LSWriteInfo>&& aWriteInfos) {
+  return Checkpoint(std::move(aWriteInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvAsyncCheckpointAndNotify(
+    nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) {
+  return CheckpointAndNotify(std::move(aWriteAndNotifyInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvSyncCheckpoint(
+    nsTArray<LSWriteInfo>&& aWriteInfos) {
+  return Checkpoint(std::move(aWriteInfos));
+}
+
+mozilla::ipc::IPCResult Snapshot::RecvSyncCheckpointAndNotify(
+    nsTArray<LSWriteAndNotifyInfo>&& aWriteAndNotifyInfos) {
+  return CheckpointAndNotify(std::move(aWriteAndNotifyInfos));
+}
+
 mozilla::ipc::IPCResult Snapshot::RecvAsyncFinish() {
   AssertIsOnBackgroundThread();
 
@@ -7889,6 +7941,18 @@ bool LSSimpleRequestBase::VerifyRequestParams() {
       break;
     }
 
+    case LSSimpleRequestParams::TLSSimpleRequestGetStateParams: {
+      const LSSimpleRequestGetStateParams& params =
+          mParams.get_LSSimpleRequestGetStateParams();
+
+      if (NS_WARN_IF(!VerifyPrincipalInfo(
+              params.principalInfo(), params.storagePrincipalInfo(), false))) {
+        return false;
+      }
+
+      break;
+    }
+
     default:
       MOZ_CRASH("Should never get here!");
   }
@@ -8043,6 +8107,60 @@ void PreloadedOp::GetResponse(LSSimpleRequestResponse& aResponse) {
   aResponse = preloadedResponse;
 }
 
+/*******************************************************************************
+ * GetStateOp
+ ******************************************************************************/
+
+GetStateOp::GetStateOp(const LSSimpleRequestParams& aParams,
+                       const Maybe<ContentParentId>& aContentParentId)
+    : LSSimpleRequestBase(aParams, aContentParentId) {
+  MOZ_ASSERT(aParams.type() ==
+             LSSimpleRequestParams::TLSSimpleRequestGetStateParams);
+}
+
+nsresult GetStateOp::Start() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::StartingRequest);
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  const LSSimpleRequestGetStateParams& params =
+      mParams.get_LSSimpleRequestGetStateParams();
+
+  const PrincipalInfo& storagePrincipalInfo = params.storagePrincipalInfo();
+
+  MOZ_ASSERT(
+      storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo ||
+      storagePrincipalInfo.type() == PrincipalInfo::TContentPrincipalInfo);
+  mOrigin = storagePrincipalInfo.type() == PrincipalInfo::TSystemPrincipalInfo
+                ? nsCString{QuotaManager::GetOriginForChrome()}
+                : QuotaManager::GetOriginFromValidatedPrincipalInfo(
+                      storagePrincipalInfo);
+
+  mState = State::SendingResults;
+  MOZ_ALWAYS_SUCCEEDS(OwningEventTarget()->Dispatch(this, NS_DISPATCH_NORMAL));
+
+  return NS_OK;
+}
+
+void GetStateOp::GetResponse(LSSimpleRequestResponse& aResponse) {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mState == State::SendingResults);
+  MOZ_ASSERT(NS_SUCCEEDED(ResultCode()));
+  MOZ_ASSERT(!QuotaClient::IsShuttingDownOnBackgroundThread());
+  MOZ_ASSERT(MayProceed());
+
+  LSSimpleRequestGetStateResponse getStateResponse;
+
+  if (RefPtr<Datastore> datastore = GetDatastore(mOrigin)) {
+    if (!datastore->IsClosed()) {
+      getStateResponse.itemInfos() = datastore->GetOrderedItems().Clone();
+    }
+  }
+
+  aResponse = getStateResponse;
+}
+
 /*******************************************************************************
  * ArchivedOriginScope
  ******************************************************************************/

dom/localstorage/LSDatabase.cpp

@@ -300,6 +300,21 @@ nsresult LSDatabase::BeginExplicitSnapshot(LSObject* aObject) {
   return NS_OK;
 }
 
+nsresult LSDatabase::CheckpointExplicitSnapshot() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(!mAllowedToClose);
+  MOZ_ASSERT(mSnapshot);
+  MOZ_ASSERT(mSnapshot->Explicit());
+
+  nsresult rv = mSnapshot->ExplicitCheckpoint();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
 nsresult LSDatabase::EndExplicitSnapshot() {
   AssertIsOnOwningThread();
   MOZ_ASSERT(mActor);
@@ -307,7 +322,7 @@ nsresult LSDatabase::EndExplicitSnapshot() {
   MOZ_ASSERT(mSnapshot);
   MOZ_ASSERT(mSnapshot->Explicit());
 
-  nsresult rv = mSnapshot->End();
+  nsresult rv = mSnapshot->ExplicitEnd();
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return rv;
   }

dom/localstorage/LSDatabase.h

@@ -84,6 +84,8 @@ class LSDatabase final {
 
   nsresult BeginExplicitSnapshot(LSObject* aObject);
 
+  nsresult CheckpointExplicitSnapshot();
+
   nsresult EndExplicitSnapshot();
 
   bool HasSnapshot() const;

dom/localstorage/LSObject.cpp

@@ -835,6 +835,29 @@ void LSObject::BeginExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
   mInExplicitSnapshot = true;
 }
 
+void LSObject::CheckpointExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                          ErrorResult& aError) {
+  AssertIsOnOwningThread();
+
+  if (!CanUseStorage(aSubjectPrincipal)) {
+    aError.Throw(NS_ERROR_DOM_SECURITY_ERR);
+    return;
+  }
+
+  if (!mInExplicitSnapshot) {
+    aError.Throw(NS_ERROR_NOT_INITIALIZED);
+    return;
+  }
+
+  AssertExplicitSnapshotInvariants(*this);
+
+  nsresult rv = mDatabase->CheckpointExplicitSnapshot();
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aError.Throw(rv);
+    return;
+  }
+}
+
 void LSObject::EndExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
                                    ErrorResult& aError) {
   AssertIsOnOwningThread();

dom/localstorage/LSObject.h

@@ -179,6 +179,9 @@ class LSObject final : public Storage {
   void BeginExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
                              ErrorResult& aError) override;
 
+  void CheckpointExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                  ErrorResult& aError) override;
+
   void EndExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
                            ErrorResult& aError) override;
 

dom/localstorage/LSSnapshot.cpp

@@ -567,7 +567,24 @@ void LSSnapshot::MarkDirty() {
   }
 }
 
-nsresult LSSnapshot::End() {
+nsresult LSSnapshot::ExplicitCheckpoint() {
+  AssertIsOnOwningThread();
+  MOZ_ASSERT(mActor);
+  MOZ_ASSERT(mExplicit);
+  MOZ_ASSERT(!mHasPendingStableStateCallback);
+  MOZ_ASSERT(!mHasPendingIdleTimerCallback);
+  MOZ_ASSERT(mInitialized);
+  MOZ_ASSERT(!mSentFinish);
+
+  nsresult rv = Checkpoint(/* aSync */ true);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  return NS_OK;
+}
+
+nsresult LSSnapshot::ExplicitEnd() {
   AssertIsOnOwningThread();
   MOZ_ASSERT(mActor);
   MOZ_ASSERT(mExplicit);
@@ -920,7 +937,7 @@ nsresult LSSnapshot::UpdateUsage(int64_t aDelta) {
   return NS_OK;
 }
 
-nsresult LSSnapshot::Checkpoint() {
+nsresult LSSnapshot::Checkpoint(bool aSync) {
   AssertIsOnOwningThread();
   MOZ_ASSERT(mActor);
   MOZ_ASSERT(mInitialized);
@@ -930,8 +947,13 @@ nsresult LSSnapshot::Checkpoint() {
     MOZ_ASSERT(mWriteAndNotifyInfos);
 
     if (!mWriteAndNotifyInfos->IsEmpty()) {
-      MOZ_ALWAYS_TRUE(
+      if (aSync) {
-          mActor->SendAsyncCheckpointAndNotify(*mWriteAndNotifyInfos));
+        MOZ_ALWAYS_TRUE(
+            mActor->SendSyncCheckpointAndNotify(*mWriteAndNotifyInfos));
+      } else {
+        MOZ_ALWAYS_TRUE(
+            mActor->SendAsyncCheckpointAndNotify(*mWriteAndNotifyInfos));
+      }
 
       mWriteAndNotifyInfos->Clear();
     }
@@ -944,7 +966,11 @@ nsresult LSSnapshot::Checkpoint() {
 
       MOZ_ASSERT(!writeInfos.IsEmpty());
 
-      MOZ_ALWAYS_TRUE(mActor->SendAsyncCheckpoint(writeInfos));
+      if (aSync) {
+        MOZ_ALWAYS_TRUE(mActor->SendSyncCheckpoint(writeInfos));
+      } else {
+        MOZ_ALWAYS_TRUE(mActor->SendAsyncCheckpoint(writeInfos));
+      }
 
       mWriteOptimizer->Reset();
     }
@@ -1035,7 +1061,7 @@ LSSnapshot::Run() {
   if (mDirty || mHasOtherProcessDatabases ||
       !Preferences::GetBool("dom.storage.snapshot_reusing")) {
     MOZ_ALWAYS_SUCCEEDS(Finish());
-  } else if (!mExplicit) {
+  } else {
     MOZ_ASSERT(mIdleTimer);
 
     MOZ_ALWAYS_SUCCEEDS(mIdleTimer->InitWithNamedFuncCallback(

dom/localstorage/LSSnapshot.h

@@ -156,7 +156,9 @@ class LSSnapshot final : public nsIRunnable {
 
   void MarkDirty();
 
-  nsresult End();
+  nsresult ExplicitCheckpoint();
+
+  nsresult ExplicitEnd();
 
   int64_t GetUsage() const;
 
@@ -175,7 +177,7 @@ class LSSnapshot final : public nsIRunnable {
 
   nsresult UpdateUsage(int64_t aDelta);
 
-  nsresult Checkpoint();
+  nsresult Checkpoint(bool aSync = false);
 
   nsresult Finish(bool aSync = false);
 

dom/localstorage/LocalStorageManager2.cpp

@@ -148,6 +148,8 @@ class SimpleRequestResolver final : public LSSimpleRequestChildCallback {
 
   void HandleResponse(bool aResponse);
 
+  void HandleResponse(const nsTArray<LSItemInfo>& aResponse);
+
   // LSRequestChildCallback
   void OnResponse(const LSSimpleRequestResponse& aResponse) override;
 };
@@ -385,6 +387,37 @@ LocalStorageManager2::IsPreloaded(nsIPrincipal* aPrincipal, JSContext* aContext,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+LocalStorageManager2::GetState(nsIPrincipal* aPrincipal, JSContext* aContext,
+                               Promise** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(_retval);
+
+  RefPtr<Promise> promise;
+  nsresult rv = CreatePromise(aContext, getter_AddRefs(promise));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  LSSimpleRequestGetStateParams params;
+
+  rv = CheckedPrincipalToPrincipalInfo(aPrincipal, params.principalInfo());
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  params.storagePrincipalInfo() = params.principalInfo();
+
+  rv = StartSimpleRequest(promise, params);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  promise.forget(_retval);
+  return NS_OK;
+}
+
 LSRequestChild* LocalStorageManager2::StartRequest(
     const LSRequestParams& aParams, LSRequestChildCallback* aCallback) {
   AssertIsOnDOMFileThread();
@@ -565,6 +598,42 @@ void SimpleRequestResolver::HandleResponse(bool aResponse) {
   mPromise->MaybeResolve(aResponse);
 }
 
+[[nodiscard]] static bool ToJSValue(JSContext* aCx,
+                                    const nsTArray<LSItemInfo>& aArgument,
+                                    JS::MutableHandle<JS::Value> aValue) {
+  JS::RootedObject obj(aCx, JS_NewPlainObject(aCx));
+  if (!obj) {
+    return false;
+  }
+
+  for (size_t i = 0; i < aArgument.Length(); ++i) {
+    const LSItemInfo& itemInfo = aArgument[i];
+
+    const nsString& key = itemInfo.key();
+
+    JS::RootedValue value(aCx);
+    if (!ToJSValue(aCx, itemInfo.value().AsString(), &value)) {
+      return false;
+    }
+
+    if (!JS_DefineUCProperty(aCx, obj, key.BeginReading(), key.Length(), value,
+                             JSPROP_ENUMERATE)) {
+      return false;
+    }
+  }
+
+  aValue.setObject(*obj);
+  return true;
+}
+
+void SimpleRequestResolver::HandleResponse(
+    const nsTArray<LSItemInfo>& aResponse) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mPromise);
+
+  mPromise->MaybeResolve(aResponse);
+}
+
 void SimpleRequestResolver::OnResponse(
     const LSSimpleRequestResponse& aResponse) {
   MOZ_ASSERT(NS_IsMainThread());
@@ -579,6 +648,11 @@ void SimpleRequestResolver::OnResponse(
           aResponse.get_LSSimpleRequestPreloadedResponse().preloaded());
       break;
 
+    case LSSimpleRequestResponse::TLSSimpleRequestGetStateResponse:
+      HandleResponse(
+          aResponse.get_LSSimpleRequestGetStateResponse().itemInfos());
+      break;
+
     default:
       MOZ_CRASH("Unknown response type!");
   }

dom/localstorage/PBackgroundLSSharedTypes.ipdlh

@@ -68,9 +68,16 @@ struct LSSimpleRequestPreloadedParams
   PrincipalInfo storagePrincipalInfo;
 };
 
+struct LSSimpleRequestGetStateParams
+{
+  PrincipalInfo principalInfo;
+  PrincipalInfo storagePrincipalInfo;
+};
+
 union LSSimpleRequestParams
 {
   LSSimpleRequestPreloadedParams;
+  LSSimpleRequestGetStateParams;
 };
 
 /**

dom/localstorage/PBackgroundLSSimpleRequest.ipdl

@@ -4,6 +4,8 @@
 
 include protocol PBackground;
 
+include PBackgroundLSSharedTypes;
+
 namespace mozilla {
 namespace dom {
 
@@ -16,6 +18,11 @@ struct LSSimpleRequestPreloadedResponse
   bool preloaded;
 };
 
+struct LSSimpleRequestGetStateResponse
+{
+  LSItemInfo[] itemInfos;
+};
+
 /**
  * Discriminated union which can contain an error code (`nsresult`) or
  * particular simple request response.
@@ -24,6 +31,7 @@ union LSSimpleRequestResponse
 {
   nsresult;
   LSSimpleRequestPreloadedResponse;
+  LSSimpleRequestGetStateResponse;
 };
 
 /**

dom/localstorage/PBackgroundLSSnapshot.ipdl

@@ -75,6 +75,14 @@ parent:
 
   async AsyncCheckpointAndNotify(LSWriteAndNotifyInfo[] writeAndNotifyInfos);
 
+  // A synchronous checkpoint. This should only be used by the snapshotting code
+  // to checkpoint an explicit snapshot.
+  sync SyncCheckpoint(LSWriteInfo[] writeInfos);
+
+  // A synchronous checkpoint and notify. This should only be used by the
+  // snapshotting code to checkpoint and notify an explicit snapshot.
+  sync SyncCheckpointAndNotify(LSWriteAndNotifyInfo[] writeAndNotifyInfos);
+
   async AsyncFinish();
 
   // A synchronous finish. This should only be used by the snapshotting code to

dom/localstorage/nsILocalStorageManager.idl

@@ -32,4 +32,7 @@ interface nsILocalStorageManager : nsISupports
 
   [implicit_jscontext]
   Promise isPreloaded(in nsIPrincipal aPrincipal);
+
+  [implicit_jscontext]
+  Promise getState(in nsIPrincipal aPrincipal);
 };

dom/storage/LocalStorageManager.cpp

@@ -318,6 +318,16 @@ LocalStorageManager::IsPreloaded(nsIPrincipal* aPrincipal, JSContext* aContext,
   return NS_ERROR_NOT_IMPLEMENTED;
 }
 
+NS_IMETHODIMP
+LocalStorageManager::GetState(nsIPrincipal* aPrincipal, JSContext* aContext,
+                              Promise** _retval) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPrincipal);
+  MOZ_ASSERT(_retval);
+
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 void LocalStorageManager::ClearCaches(uint32_t aUnloadFlags,
                                       const OriginAttributesPattern& aPattern,
                                       const nsACString& aOriginScope) {

dom/storage/Storage.h

@@ -118,6 +118,9 @@ class Storage : public nsISupports, public nsWrapperCache {
   virtual void BeginExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
                                      ErrorResult& aRv) {}
 
+  virtual void CheckpointExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
+                                          ErrorResult& aRv) {}
+
   virtual void EndExplicitSnapshot(nsIPrincipal& aSubjectPrincipal,
                                    ErrorResult& aRv) {}
 

dom/tests/browser/browser_localStorage_snapshotting.js

@@ -71,6 +71,12 @@ async function beginExplicitSnapshot(knownTab) {
   });
 }
 
+async function checkpointExplicitSnapshot(knownTab) {
+  await SpecialPowers.spawn(knownTab.tab.linkedBrowser, [], function() {
+    return content.wrappedJSObject.checkpointExplicitSnapshot();
+  });
+}
+
 async function endExplicitSnapshot(knownTab) {
   await SpecialPowers.spawn(knownTab.tab.linkedBrowser, [], function() {
     return content.wrappedJSObject.endExplicitSnapshot();
@@ -99,6 +105,24 @@ async function verifySnapshotUsage(knownTab, expectedSnapshotUsage) {
   is(snapshotUsage, expectedSnapshotUsage, "Correct snapshot usage");
 }
 
+async function verifyParentState(expectedState) {
+  let principal = Services.scriptSecurityManager.createContentPrincipalFromOrigin(
+    HELPER_PAGE_ORIGIN
+  );
+
+  let actualState = await Services.domStorageManager.getState(principal);
+
+  for (let [expectedKey, expectedValue] of Object.entries(expectedState)) {
+    ok(actualState.hasOwnProperty(expectedKey), "key present: " + expectedKey);
+    is(actualState[expectedKey], expectedValue, "value correct");
+  }
+  for (let actualKey of Object.keys(actualState)) {
+    if (!expectedState.hasOwnProperty(actualKey)) {
+      ok(false, "actual state has key it shouldn't have: " + actualKey);
+    }
+  }
+}
+
 // We spin up a ton of child processes.
 requestLongerTimeout(4);
 
@@ -648,3 +672,84 @@ add_task(async function() {
 
   clearOriginStorageEnsuringNoPreload(HELPER_PAGE_ORIGIN);
 });
+
+/**
+ * Verify that datastore in the parent is correctly updated after a checkpoint.
+ */
+add_task(async function() {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // Force multiple web and webIsolated content processes so that the
+      // multi-e10s logic works correctly.
+      ["dom.ipc.processCount", 4],
+      ["dom.ipc.processCount.webIsolated", 2],
+      // Enable LocalStorage's testing API so we can explicitly create
+      // snapshots when needed.
+      ["dom.storage.testing", true],
+    ],
+  });
+
+  // Ensure that there is no localstorage data by forcing the origin to be
+  // cleared prior to the start of our test.
+  await clearOriginStorageEnsuringNoPreload(HELPER_PAGE_ORIGIN);
+
+  // Open tabs.  Don't configure any of them yet.
+  const knownTabs = new KnownTabs();
+  const writerTab1 = await openTestTab(
+    HELPER_PAGE_URL,
+    "writer1",
+    knownTabs,
+    true
+  );
+
+  await verifyParentState({});
+
+  // Apply the initial mutation using an explicit snapshot. The explicit
+  // snapshot here ensures that the parent process have received the changes.
+  await beginExplicitSnapshot(writerTab1);
+  await verifyParentState({});
+  await applyMutations(writerTab1, [["key", "something"]]);
+  await verifyParentState({});
+  await endExplicitSnapshot(writerTab1);
+
+  await verifyParentState({ key: "something" });
+
+  // Begin an explicit snapshot in writerTab1, apply the first mutation in
+  // writerTab1 and checkpoint the explicit snapshot.
+  await beginExplicitSnapshot(writerTab1);
+  await verifyParentState({ key: "something" });
+  await applyMutations(writerTab1, [["key", "somethingBigger"]]);
+  await verifyParentState({ key: "something" });
+  await checkpointExplicitSnapshot(writerTab1);
+
+  await verifyParentState({ key: "somethingBigger" });
+
+  // Apply the second mutation in writerTab1 and checkpoint the explicit
+  // snapshot.
+  await applyMutations(writerTab1, [["key", null]]);
+  await verifyParentState({ key: "somethingBigger" });
+  await checkpointExplicitSnapshot(writerTab1);
+
+  await verifyParentState({});
+
+  // Apply the third mutation in writerTab1 and end the explicit snapshot.
+  await applyMutations(writerTab1, [["otherKey", "something"]]);
+  await verifyParentState({});
+  await endExplicitSnapshot(writerTab1);
+
+  await verifyParentState({ otherKey: "something" });
+
+  // Verify the final state, it should match the state after the third mutation
+  // has been applied and "commited". An explicit snapshot is used.
+  await beginExplicitSnapshot(writerTab1);
+  await verifyParentState({ otherKey: "something" });
+  await verifyState(writerTab1, { otherKey: "something" });
+  await endExplicitSnapshot(writerTab1);
+
+  await verifyParentState({ otherKey: "something" });
+
+  // Clean up.
+  await cleanupTabs(knownTabs);
+
+  clearOriginStorageEnsuringNoPreload(HELPER_PAGE_ORIGIN);
+});

dom/tests/browser/page_localstorage_snapshotting.html

@@ -46,6 +46,10 @@ function beginExplicitSnapshot() {
   localStorage.beginExplicitSnapshot();
 }
 
+function checkpointExplicitSnapshot() {
+  localStorage.checkpointExplicitSnapshot();
+}
+
 function endExplicitSnapshot() {
   localStorage.endExplicitSnapshot();
 }

dom/webidl/Storage.webidl

@@ -65,6 +65,15 @@ partial interface Storage {
   [Throws, NeedsSubjectPrincipal, Pref="dom.storage.testing"]
   void beginExplicitSnapshot();
 
+  /**
+   * Checkpoints the explicitly begun snapshot. This is only useful for testing
+   * of snapshot re-using when multiple checkpoints are involved. There's no
+   * need to call this before `endExplicitSnapshot` because it checkpoints the
+   * snapshot before it's ended.
+   */
+  [Throws, NeedsSubjectPrincipal, Pref="dom.storage.testing"]
+  void checkpointExplicitSnapshot();
+
   /**
    * Ends the explicitly begun snapshot and retains the underlying database.
    * Compare with `close` which also drops the reference to the database.

ipc/ipdl/sync-messages.ini

@@ -854,6 +854,10 @@ description = legacy sync IPC - please add detailed description
 description = legacy sync IPC - please add detailed description
 [PBackgroundLSDatabase::PBackgroundLSSnapshot]
 description = See corresponding comment in PBackgroundLSDatabase.ipdl
+[PBackgroundLSSnapshot::SyncCheckpoint]
+description = See corresponding comment in PBackgroundLSSnapshot.ipdl
+[PBackgroundLSSnapshot::SyncCheckpointAndNotify]
+description = See corresponding comment in PBackgroundLSSnapshot.ipdl
 [PBackgroundLSSnapshot::SyncFinish]
 description = See corresponding comment in PBackgroundLSSnapshot.ipdl
 [PBackgroundLSSnapshot::LoadValueAndMoreItems]